Cocciodiosis is an avian disease caused by infection with one or more of the many species of coccidia, which are intracellular protozoal parasites of the subphylum Apicomplexa and the genus Eimeria. Cocciodiosis is known to be caused by several different species of Eimeria, namely Eimeria acervulina, E. maxima, E. tenella, E. necatrix, E. brunetti, E. mitis, E. praecox, and possibly E. mivati and E. hagani. The species do differ in their pathogenic effect on birds, with the type of birds also playing a role. Thus, a broiler chicken will be subjected to a great deal of damage by a parasite such as E. acervulina because it infects large portions of the small intestine, where food digestion plays a major role.
During its life cycle, the Eimeria parasite passes through a number of stages. The life cycle begins when the chicken ingests the infectious stage, known as the sporulating oocyst, during ground feeding or by inhalation of dust. The wall of the sporulated oocyst is ruptured by the mechanical action in the gizzard and intestinal tract, resulting in the release of four sporocysts. The sporocysts pass into the duodenum where they are exposed to bile and digestive enzymes resulting in the release of an average of two sporozoites per sporocyst.
The sporozoites are mobile and search for suitable host epithelium cells in order to penetrate and reproduce in them. Following infection of an epithelium cell, the parasite enters the schizont phase of its life cycle, producing from 8 to 16 to greater than 200 merozoites per schizont. Once released from the schizont, the merozoites are free to infect further epithelium. After from two to five of these asexual reproduction cycles, the intracellular merozoites grow into sexual forms known as the female or macrogametocyte and the male or microgametocyte. Following fertilization of the macrogametocyte by the microgametes released from the microgametocyte, a zygote is formed which creates a cyst wall about itself. The newly formed oocyst is passed out of the infected chicken with the droppings.
With the correct environmental conditions of temperature and humidity and sufficient oxygen in the air, the oocyst will sporulate into the infectious stage, ready to infect a new host and thereby spread the disease. Thus, no intermediate host is required for transfer of the parasite from bird to bird.
The result of the Eimeria parasite infecting the digestive tract of a bird may be a reduction in weight gain, decreased feed conversion, cessation of egg production and, in some cases, death. The increase in intensive production of poultry has been accompanied by severe losses due to this parasite. It is estimated that losses in the United States and Europe exceed several hundred million dollars annually.
Several attempts have now been made to control Coccidiosis. Prior to the advent of chemotherapeutic agents, improved sanitation using disinfectants, together with mechanical removal of litter, were the main methods employed. In addition, the introduction of coccidiostatic agents in the feed or drinking water, along with good management practices, has resulted in some success at disease control. However, such agents have been found to suffer from a drop in effectiveness over the years, due partly to the development of drug resistant strains of coccidia. Furthermore, several chemotherapeutic agents have been found to leave residues in the meat of commercial birds, thereby often making it unsuitable for consumption.
Other attempts to control the disease have been made immunologically. These have included the development of live vaccines and the use of genetic engineering to formulate vaccines. Many of these efforts have focused on the use of Eimeria proteins as the active component of the vaccine. For example, Tomley et al. in U.S. Pat. Nos. 5,885,568 and 6,001,363 propose an Eimeria protein with immunogenic properties, as well as DNA sequences which encode these proteins. A similar approach is set forth in Kok et al., U.S. Pat. No. 6,100,241. A DNA molecule having a particular nucleic acid sequence is set forth in Andrews et al., U.S. Pat. No.s 4,874,705 and 5,187,080, while Mewman et al., U.S. Pat. No. 5,028,694, describes a purified antigenic protein which is capable of inducing an immune response in a chicken against Eimeria necatrix or Eimeria tenella. A vaccine against Coccidiosis is also described in Brown et al., U.S. Pat. No. 6,019,985.
What is now needed in the art is a new vaccine against Coccidiosis which is safe, effective, easy to administer and cost efficient. Also needed are new strains of Eimeria species which can be used in eliciting an immune response in poultry, so as to provide suitable oocysts for a vaccine. Further needed are new methods of attenuating strains of Eimeria so that they can be utilized for vaccine development. The art further necessitates the development of strains of Eimeria whose oocysts, when incorporated into a vaccine, are cross-protective against a wider sampling of Eimeria species.